1. Field of the Invention
The present invention relates to filler materials for adding to various matrix materials, and capacitors, especially for making electrodes for electric double-layer capacitors, secondary batteries, especially lithium- and lead-based secondary batteries.
2. Description of the Related Art
Carbon fibers produced in vapor phase (vapor grown carbon fibers) are added as a filler agent to various polymeric materials, organometallic materials as well as metals and ceramics to improve their properties such as electrical conductivity, frictional characteristics, thermal conductivity and mechanical strength.
In general, vapor grown carbon fibers (VGCF) are available in dimensions of 0.05xcx9c5 xcexcm diameter and 1xcx9c1,000 xcexcm length, and their features include well developed graphitic cell surfaces along the fiber axis and hollow interior. In the as-grown condition, these micro-fibers assume a cluster form of a low apparent density.
In general, carbonaceous substances such as tar and pitch remain in such fiber clusters, which can be carbonized by heating and such converted materials are removed as necessary. This heating process improves the properties of the carbon fibers thus produced.
Specific heating procedures for such vapor grown carbon fibers include, for example, a method based on continuous or intermittent heating of carbon fibers in a heat resistant container (Japanese Patent Application, First Publication (A) , H1-272827); a method based on kneading fiber clusters and heating (Japanese Patent Application, First Publication (A) , H1-270543); and a method based on heating the carbon fibers molded into certain shapes (Japanese Patent Application, First Publication (A), H1-290570) When the carbon fiber clusters are to be used as a filler in high molecular weight materials, it is necessary that the fiber clusters be ground to a powder. A specific example includes grinding the carbon fiber clusters minutely with the use of a jet mill (Japanese Patent Application, First Publication (A), S63-21208 and Japanese Patent Application, First Publication (A) , S63-283766); and highspeed rolling in a grinding mixture of balls of less than 10 mm diameter, made of ceramics such as zirconia and alumina and hard metals (Japanese Patent Application, First Publication (A), H6-32607). A method based on grinding the singular fibers themselves is also disclosed (Japanese Patent Application, First Publication (A), H4-222227).
Although the vapor grown carbon fibers themselves exhibit good electrical and thermal conductivity properties, the present state of the art is such that when they are used as a filler material in making polymeric composite products, their favorable electrical and thermal conductive are not reflected in the properties of the composite product made therewith.
In considering a case of improving the electrical conductivity of a composite product, the filler should be generally in a form of fibers because it is believed that the conductivity paths should be made long.
However, the longer the fibers the higher the tendency for the fibers to orient in a preferred direction, and a resin mixture having such a fiber alignment depresses the fluidity and consequently the formability of the composite feed.
The result is that the present practice is to use whisker fibers that are microscopic such as those as-grown fibers or milled singular fibers made therefrom. Especially, it is considered that preferred fibers are singular fibers that have aspect ratios of greater than 10 preferably more than 100 and have the length dimensions in the range of several tens of microns.
However, as the fibers become smaller, the number of contact points of the fibers also increases and the electrical contact resistance is increased. Furthermore, contact points of the fibers are vulnerable to infiltration by the matrix resin during molding, and the electrical conductivity of the composite product is ultimately degraded.
Regarding the method of heating the carbon fibers in a heat resistant container or the method based on kneading fiber clusters and heating mentioned above, the difficulty has been that the density of the fiber cluster is low because of insufficient pressure, so that not only the fiber contact points are few but bonding at the fiber contact points is insufficient.
Regarding the method of grinding the fibers minutely, the difficulty has been that, because the fibers are milled too fine, too many fiber contact points are created, which increase the electrical contact resistance. When such a composite feed is. formed into a product, the fiber contact points are broken by the infiltrated resin during the molding process, resulting that the electrical conductivity cannot be increased.
The above Japanese Patent Application, First Publication (A), H1-290570 discloses a method of forming and sintering of the fibers, but the fibers are meant to be used for thermal insulation, and are not for use as a filler material.
Therefore, it can be understood that the VGCF at the present time are not able to demonstrate their full potential in improving the electrical and thermal conductivity characteristics of composite materials.
Fibrous fillers themselves exhibit an effective electrical conductivity in two-dimensions because of their lengthwise geometry, but, in a composite material, it is required that the electrical conductivity property be improved in three-dimensions. It follows that the fibrous material for making a composite product should have a structure such that the properties are isotropic in three-dimensions and that the conductivity path be as long as possible. It may be considered that, to simply increase the path lengths in three-dimensions, larger particles would be preferable. However, large particles are not suitable as a filler material because, when added to a matrix, such large particles not only diminish the contact point per unit volume of the matrix but adversely affect the formability and strength properties of the composite product.
Therefore, to increase the electrical conductivity in a composite product while retaining the strength and formability properties of the composite material, it is desirable to have a fibrous structure that creates a functional three-dimensional structure with a high number of contact points which retains the integrity of the electrical conductive paths.
Such a filler material comprised by vapor grown carbon fibers and has an ideal fiber structure is not available currently.
In recent years, there have been developments and commercialization of charge storage devices based on electric double-layers, that is, electric double-layer capacitors derived from the electric double-layer principle. Because of high static capacitance achievable in electric double-layer capacitors, in small size capacitors are suitable as backup electrical source for semiconductor memories in electronic apparatus while large devices are even starting to be used as a part of lead storage batteries for vehicular power applications.
Generally speaking, carbon electrodes are used in electric double-layer capacitors. These electric double-layer capacitors utilize electric double-layers formed near the interfaces between the electrode surface and electrolyte, therefore, it is necessary to increase the surface area of the carbon electrodes.
For this reason, various electrode designs have been proposed such as, for example, carbon electrodes made of activated charcoal particles having micro-porosities (Japanese Patent Application, First Publication (A), S63-18761: and Japanese Patent Application, First Publication (A), H1-227417); carbon electrodes produced by heating a polyvinylidene chloride resin in a non-oxidizing atmosphere so as to generate atomic and molecular faults to form micro-porosities (Japanese Patent Application, First Publication (A), H7-249551).
On the other hand, electrode plates of high capacitance require not only high electrode surface area (i.e. specific surface area) but also high electrical and thermal conductivities.
However, the activated charcoal particles mentioned above have low crystallinity and the electrical resistance of the material itself is high, and it is difficult to obtain sufficient electrical conductivity from the electrodes made from such activated charcoal particles because of the high inter-particle electrical contact resistance.
For this reason, there has been a proposal for an electric double-layer capacitor which used electrodes having activated charcoal particles and carbon whiskers (Japanese Patent Application, First Publication (A), H7-307250).
However, although a filler material containing singular form of fiber carbon whiskers provides two-dimensional conductance effects, as discussed above, it does not provide sufficient conductance in three-dimensions. It is better and preferable that the electrode plates for electric double-layer capacitor be uniform in their overall three-dimensional properties.
To improve the electrical conductivity, a filler material should have a structure exhibiting isotropic properties in three dimensions and provide as long a conductive distance as possible, and additionally, it is preferable that the filler material should have a web-like structure that enables to support sufficient Quantity of activated charcoal particles.
However, at the present time, there is no electrode material to fulfill such requirements for making high performance electric double-layer capacitors.
Some secondary batteries, in particular lithium- or lead-based secondary batteries, use electrodes containing carbon fibers, especially VGCF.
For example, negative electrodes, in lithium ion secondary batteries, made from a mixture of VGCF and graphite powder obtained by high temperature processing of cokes are known (Japanese Patent Application, First Publication (A), H4-155776).
Also, negative electrodes in lithium ion secondary batteries made from a mixture of meso-carbon microbeads and VGCF are known (Japanese Patent Application, First Publication (A), H4-237971).
These lithium secondary batteries are said to prevent expansion of the electrode active material such as graphite powder, thus suppressing grid deformation and fracture generated during repeated charge/discharge cycles, thereby resisting degradation of their charge storage capacity.
However, in such conventional electrode materials, because the graphite powder and like others and VGCF exist simply in a mechanically blended form, the carbon fibers are not able to sufficiently suppress destruction of the electrodes by grid expansion, and it is insufficient to prevent the consequent loss of capacity degradation.
It is an object of the present invention to provide a carbon fiber material that can be used as a filler material to improve thermal and electrical conductivities of a composite product containing the carbon fibers.
To effectively utilize the properties of the carbon fibers to improve the electrical and thermal conductivities of a composite product, it is desirable that the filler material provide a functional three-dimensional structure such that the fibers, which are longer than singular whisker fibers, are bonded to provide bridging paths for the fibers. The bulk density should be made as high as possible because the higher the density the greater the number of fiber contact points and higher the conductivity.
Examinations of various methods for making carbon fiber material having a functional three-dimensional structure, from singular or branching fibers made by vapor phase growth methods, have led to a discovery that such a material can be made as follows. Carbon fiber clusters are first press molded to produce a fiber compact to increase the fiber contact points, and the pressed compact is heat treated to bond the fibers by carbonizing the contact points, the heated compacts are then pulverized to obtain a material comprised substantially of carbon fiber agglomerates that are comprised by flocs-like fibers.
The carbon fiber material thus produced is comprised substantially by an agglomerated structure of fibers, having agglomerate sizes in a range of not less than 5 xcexcm and not more than 500 xcexcm, formed by vapor grown carbon fibers having a diameter of not less than 0.05 xcexcm and not more than 5 xcexcm wherein at least a fraction of fiber contact points are bonded together with substances produced by carbonizing carbonaceous matters.
The carbon fiber material is produced by a method comprising the steps of: making carbon fibers having a diameter of not less than 0.05 xcexcm and not more than 5 xcexcm by a vapor phase growth method; press forming of carbon fibers obtained to produce compacts having an apparent density of not less than 0.02 g/cm3; heating the compacts at a temperature of not less than 600xc2x0 C., preferably not less than 800xc2x0 C.; pulverizing heat treated compacts by shear or compressive forces to produce fiber agglomerates having agglomerate sizes ranging from not less than 5 xcexcm and not more than 500 xcexcm.
It is desirable that the fiber compacts be press formed at a compaction pressure of not less than 0.1 kg/cm2.
The carbon fiber material of the present invention is comprised substantially by agglomerated structures of vapor grown micro-fibers, resembling flocs, which are strongly bonded at the fiber contact points. The spatial continuity of the structure promotes a superior electrical conductivity because of the functional three dimensional fiber structure. The agglomerates are limited in their sizes and exhibit excellent dispersive properties when they are used as a filler material in a polymeric resin. Because the fibers are bonded, the structural integrity is maintained even when they are made into a composite product, thus leading to superior electrical conductivity in the composite product.
An application of the carbon fiber material is an electrode material for an electric double-layer capacitor comprised by a carbon powder having a specific surface area of not less than 500 m2/g and carbon fibers in a range of 0.1 weight percent to 30 weight percent, wherein the carbon fibers are comprised substantially by fiber agglomerates, comprised by flocs-like fibers, having agglomerate sizes in a range of not less than 5 xcexcm and not more than 500 xcexcm, formed by vapor grown carbon fibers having a diameter of not less than 0.5 xcexcm and not more than 5 xcexcm wherein at least a fraction of fiber contact points are bonded together with substances produced by carbonizing carbonaceous matters.
In such an electrode material, activated charcoal powder which is the electrode active material is incorporated into the interstices of the flocculated fiber agglomerates or is enmeshed into the fiber agglomerates of branching carbon fibers so that the probability of contacts between the charcoal particles and the carbon fibers is increased, thereby leading to increased electrical conductivity. Therefore, the internal resistance is lowered and current can flow quite readily. The activated charcoal particles are thus effectively utilized for electrolyte reaction and the charge storage capacity of the condenser is increased.
Furthermore, because the charcoal particles are enmeshed into the interstices of the fiber agglomerates which resemble flocs or a web-structure, destruction of the electrode plates is prevented, by accommodating grid expansion due to heat generated during charge/discharge cycles, and preventing separation and falling out of the particles from the electrodes. The result is that the charge storage capacity of the battery is preserved and the service life is prolonged.
The electrode material for electric double-layer capacitor exhibits excellent electrical conductivity in three-dimensions because of the flocculated vapor grown carbon fibers providing spatial electrical paths in the electrodes. The capacity/volume ratio is also superior, and the charcoal particles enmeshed inside the flocculated carbon fiber grids produce high strength.
Another application of the carbon fiber material is a battery electrode material comprised by a composite material having an electrode active material included within micro-pores in fiber agglomerates formed by tangled masses of vapor grown carbon fibers. It is preferable that the agglomerate structure is produced by press forming of branching carbon fibers produced by a vapor growth technique, heating the fibers compacts and pulverizing the compacts.
In the battery electrode material, the electrode active material other than graphite powder may be used. It may be a lithium-based oxide composite, such as LiCoO2. In this case, it is preferable to also incorporate carbon powder as conductivity enhancing substances such as carbon black, graphite powder within the interstices of the agglomerate structure.
As the electrode active material, graphite powder or the like are used. Also, other electrode active materials may include powder of lead dioxide or metallic lead. In this case, the electrode material is suitable for sealed type secondary batteries.
The electrode material of the present invention is suitable for making positive and negative electrodes in lithium ion-based secondary batteries and sealed type lead-based secondary batteries.
The electrodes made by the electrode material of the present invention are durable because the electrode active material is incorporated within the interstices of the agglomerate structure such that the expansion of the grids caused by charge/discharge cycles of the electrode active material is controlled, thereby preventing deformation and destruction of the electrode.
The agglomerate structure is produced not only by mutual mechanical contact of the fibers but also by a tangled, flocculated mass of fibers which are partially bonded chemically at the fiber contact points, therefore, the electrical resistance of the agglomerate structure is low, and the internal resistance of the electrode produced therefrom is also lowered.
The electrode material of the present invention can generate a large number of electrical contacts by providing an increased probability of electrical contacts between the charcoal particles and the carbon fibers because of the special agglomerate structure produced by including the electrode active material within the interstices of the agglomerates and enmeshing the particles with the fine fibers. Also, because the agglomerate structure itself has a low resistance, the internal resistance of the battery made therewith is low, and the active material is fully utilized to produce an effective electrolytic reaction, thereby increasing the charge storage capacity of the battery.
When a graphite powder is used as the active material, grid expansion caused by charge/discharge cycles of the graphite powder is controlled, thereby preventing electrode expansion and deformation to increase the number of useful cycles, thereby increasing the service life of the battery.